Regenerated cellulose is the term given to cellulose precipitated from a chemically dissolved native cellulose. It is also referred to as cellulose II and is different from native cellulose in having a lower molecular weight (degree of polymerization), less molecular entanglement, less degree of crystallinity, as well as a modified allotropic structure of the crystalline region (unit cell having different dimensions than native cellulose).
Regenerated cellulose was first prepared by denitrating cellulose nitrate, according to U.S. Pat. No. 264,987. This process has been commercialized as described by U.S. Pat. No. 2,445,333 and U.S. Pat. No. 2,451,768. However, the most common manufacturing methods for commercial production today are the viscose and the cupramonium methods. In the first process cellulose is regenerated from cellulose xanthate solution as described in U.S. Pat. Nos. 981,368 and 991,267 and German offen No. 413,511. Regenerated cellulose membranes prepared by this process are called celluphane. The cupramonium process regenerates cellulose in a similar manner to the viscose process from its soluble copper complex formed by reacting with ammoniacal copper sulphate, according to U.S. Pat. Nos. 2,035,645 and 2,067,522. The regenerated cellulose membranes prepared by this process are called cuprophane.
The most recent method of preparing regenerated cellulose uses cupriethylene diamine to dissolve the cellulose by complex formation. The resulting solution is used to regenerate cellulose in the normal manner according to V. K. Kulshrestha and W. J. Kolff. J.P.S. No. 297, 1970. Membranes prepared this way are called cuenophane.
Other methods of dissolving cellulose by complex formation have been cited in the prior art, e.g., dissolution in SO.sub.2.sup.+ ethylene diamine+DMSO or DMSO+Paraformaldehyde at 95.degree.-100.degree. C. [see: "DMSO/Paraformaldehyde: A Nondegrading Solvent For Cellulose", Johnson, Nicholson and Haigh; Applied Polymer Symposium, Series 28 931-943, 1976; "The Solution and Regeneration of Cellulose Using Novel Solvent Systems", Guthrie and Hardcastle, Polymer 18, pg 203-04, February 1977; and "Cellulose Solvents" Turbak, et al., Chemtech, pg. 51-57, Jan. 1980].
Irrespective of the method of preparation, all forms of regenerated cellulose are essentially the same. Cuenophane is claimed to be more permeable by virtue of the fact that it has less crystalline structure. Apart from that all regenerated cellulose membranes have similar properties, with average thicknesses from &lt;5 .mu.M up to 200 .mu.M, equivalent average pore radius of about 21 .ANG. and a molecular weight cut off (MWCO) of about 10,000 to 12,000. This last parameter is a standard ultrafiltration measure denoting the size of pores capable of &gt;99% rejection of protein molecules in aqueous systems where they coil up in spherical form.
All regenerated cellulose membranes, like most other membranes, cannot be dried. If dried the membrane's internal structure collapses by intra chain hydrogen bonding. These membranes must be kept wet at all times. For example, GB Pat. No. 1,349,394 describes a method to preswell regenerated cellulose by glycerol so that the membrane can be handled dry and used directly to permeate water.
Because of its pore size, regenerated cellulose is very useful for, and has found extensive use in, hemodyalysis and hemofiltration to purify blood of toxins in the artificial kidney. This is the most relevant use of regenerated cellulose as far as this patent is concerned. Regenerated cellulose is also extensively used for fiber production as a wrapping material, in the form of beads and sponges for adsorption and in preparing molded objects.
It is also reported in the prior art that regenerated cellulose was found useless for water desalting (U.S. Government Report 907, J. K. Smith, E. Klein, Office of Saline Water Res., Washington, DC 1970) because of its poor salt rejection compared with cellulose acetate.
Regenerated cellulose membranes may be converted into a hydrophobic form and used for organic liquid separation especially separation of ketone dewaxing solvents from dewaxed oils as presented in U.S. Ser. No. 263,116 U.S. Ser. No. 263,117 and U.S. Ser. No. 263,307, (See European Patent Application Publication No. 13,834), hereby incorporated by reference. In choosing the regenerated cellulose membrane which is to be subjected to the above identified conversion process one must select that membrane having a pore size such that it will provide the desired selectivity after the conversion process. If the regenerated cellulose membrane possesses pores of too large a size, the membrane, even after treatment, may still allow all molecules in the feed stream to pass in an unhindered manner through the membrane and thus prove to be non-selective.
So, in summary, regenerated cellulose membranes are prepared by three main methods to produce very similar membranes called celluphane, cuprophane and cuenophane. All membranes have similar average pore radii of about 20 .ANG. and can vary in thickness from &lt;10 .mu.M to 200 .mu.M and must be kept wet at all times. The most important use of that membrane is in hemodyalysis, hemofiltration, and polar organic liquids separation.